Fluid tanks are used in a variety of industries to store or mix fluids. For example, fluid tanks may be used in conjunction with subterranean operations to mix fluids before they are used to treat a subterranean formation. When using fluid tanks, it is often desirable to keep track of the amount of fluid contained in the tank as fluids are added and removed. Errors in measurement of fluid quantity can cause the system to become inoperative or to change its calibration over time.
Currently, various non-contact and contact methods are used to measure the fluid quantity. However, the existing methods of measuring the quantity of fluid in a mixing tank are affected by fluid characteristics. For varying types of non-contact methods, foams, vapors, fluid color, fluid density, and surface turbulence can lead to inaccuracy of sensor readings. Similarly, for contact methods, such as those involving floats, measurements are adversely affected by gumming, particle bridging, and/or friction caused by the forces acting on the float. Moreover, float systems are vulnerable to errors resulting from fouling of the electronic or electric components induced by the necessity to operate the sensing element in direct contact with the fluid in the mixing tank.
Other traditional methods of measuring the quantity or height of a fluid in a tank involve the use of a metering rod or gauge. However, such traditional methods are inherently inaccurate due to measurement inaccuracies and precision errors and have a limited achievable resolution.
Height sensors have been used as a way to overcome the drawbacks of the traditional methods of tracking the fluid levels in a tank. Height sensors may be used to obtain a direct measurement of the height or the quantity of a fluid in a mixing tank. However, such height sensor readings are often subject to the effects of the system noise as well as the noise created by the sensor itself. Moreover, the fluid tanks are often subject to movement resulting in height oscillations which can cause errors in the height sensor readings.
An improved method for estimating the height of a fluid contained in a mixing tank using a control system to minimize the resulting noise has been disclosed in U.S. patent application Ser. No. 11/029,072 (hereinafter the “'072 application”). However, a major drawback of the improved method described in the '072 application is that the tank's specific dimensions must be fed into the system to obtain an estimate of the amount of fluid in the tank, as the fluid height is dependent on the tank's shape.
Also known in the prior art are various forms of pressure sensing methods. These methods are affected by fluid density as well as internal pressure forces. As the temperature changes within the mixing tank, significant pressures can build up causing the fluid, mixing tank and measurement devices to expand if not properly restrained. However, this method has several drawbacks. First, additional structure required to prevent such expansion adds to the cost of the measurement system. Moreover, the load cell readings are time lagged and do not provide a zero-lag estimation of the amount of fluid in the tank as fluids are added to and/or removed from the tank. Hence, errors in the load cell reading and other parts of the system have an adverse impact on the estimation of the fluid amount in the tank.
Moreover, it is often desirable to track the rate at which fluids are added to or removed from a tank. Traditionally, the volumetric rate is tracked using the differentiated average volumetric rate. In order to obtain the differentiated average volumetric rate, the volume of the fluid in the tank is measured at a first time (t1) and a second time (t2). The difference between the volume at t1 and t2 is then divided by the sample period, i.e., t1−t2, providing the rate of change of fluid volume. However, this method has several drawbacks. The measured signals at t1 and t2 contain noise which may be amplified when the signals are differentiated. Therefore, the differentiated signal must be filtered by averaging the result over many samples in order to eliminate the noise. However, the filtering process adds delays to the signal. Therefore, an operator is forced to choose between a noisy signal and a delayed signal, neither of which is desirable.